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Processing-Structure-Property Relationships in Heterogeneously Structured Metals


Classical processing of metallic materials is incorporated to selectively improve desired properties by processing that results in a homogenous microstructure; however, the improvement in one property often results in deterioration of another property. For example, the strengths of metals can be increased by grain refinement processing techniques due to grain boundary strengthening (i.e., the Hall-Petch relationship), but a significant loss of ductility is often observed to limit the applicability of these processing methods. To avoid these shortcomings, novel processing techniques are investigated to synthesize metals with spatial heterogeneity in their microstructure. The heterogeneous structure allows combinations of material properties that are not achievable by standalone homogenous counterparts. In this dissertation, I have investigated two different processing techniques for the formation of heterogeneously structured metals and explored their structure-property relationship.

Surface severe plastic deformation (S2PD) techniques such as surface mechanical attrition treatment (SMAT) allow processing of metals to form a gradient of stored deformation and microstructure to increase strengths and retain ductility. Herein, SMAT of Cu followed by annealing at temperatures of 175 °C, 225 °C, and 275 °C for one hour is performed to investigate the mechanical properties-microstructure evolution. Following the annealing, the microstructural evaluation revealed a recrystallized layer near the treated surface that grew in depth with an increase in temperature. A substantial increase in uniform elongation was observed after annealing at 225 °C and 275 °C. The results indicate the importance of a deformation-free microstructure in ductility of the S2PD processed metals.

Bulk metallic glasses (BMG) are best known for their high strength; however, due to their limited tensile plasticity, they are undesirable for most structural applications. Nanolaminated amorphous/crystalline metallic composites fabricated via deposition techniques have been shown to deform homogeneously while demonstrating extraordinary mechanical properties including high strength and ductility. However, their fabrication is limited in size and scalability potential. Herein, accumulative roll bonding (ARB) has been demonstrated as a scalable fabrication technique for the processing of nanolaminated Zr-based BMG/Ni composites. Refined BMG layers with thicknesses as small as 34 nm and an amorphous/crystalline interface with an effective interface width of 3-4nm have been characterized.

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